The present invention relates to a focussing apparatus for cameras, in which the degree of sharpness or blurring of an object image produced by the taking lens can be directly observed through the viewfinder during the focussing operation.
Most 35 mm compact cameras in use today are adapted to focus automatically on the object to be photographed in such a way that the taking lens is automatically shifted by a distance corresponding to the object distance detected or measured by an automatic built-in focussing device. There are various types of object distance measuring techniques employed in automatic focussing devices. One of these is a trigonometrical ranging technique, whilst another is an ultrasonic echo-ranging technique.
In an automatic focussing device based on the trigonometrical ranging technique, there are first and second auxiliary optical means, namely a stationary mirror and a movable mirror, for producing a pair of auxiliary or detection images on a pair of multi-element detector means. The latter means are so arranged that the distribution of the light patterns of the two images are sensed and compared. An identity of the light distributions relative to the corresponding detector means coincides with an optimum focus of the principal image produced by a taking lens. Such a device is widely known as a VISITRONIC AUTO FOCUS DEVICE (trademark of Honeywell Inc., Minneapolis, Minn., U.S.A.)
A camera with the type of built-in autofocus device described above is usually adapted to indicate a focus position of the taking lens in a viewfinder by means of a plurality of focussing zone marks and a pointer operatively connected with the taking lens, each of which marks is provided in an inverted Galilean finder. There is however the disadvantage that the indication thus obtained does not include any information other than the focus position of the taking lens. To avoid this disadvantage, there is provided a device which can show a sharp image in a viewfinder only when the taking lens has been made to focus an image on a film. This device includes an auxiliary lens which serves to indicate a distance measuring area in a viewing field and is caused to be removed from the optical path of the inverted Galilean finder only upon the completion of focussing operation.
According to the device aforementioned, the image within the distance measuring area on the viewing field is out of focus during the focussing operation because of the auxiliary lens in the optical path of finder. On the other hand, the image becomes sharp simultaneously with the completion of the focussing operation because of the removal of the auxiliary lens from the optical path.
The known device is, therefore, useful only in that it can indicate whether or not the taking lens has been made to focus an image on a film. In other words, it cannot indicate whether the image is out of focus. Therefore, in a camera adapted to focus by manually shifting the taking lens while watching an auxiliary image through the viewfinder, there is the disadvantage in operation that the photographer cannot exactly determine the shifting distance of the taking lens for focussing. Furthermore, if the taking lens is automatically shifted by a driving motor for focussing, the photographer cannot determine the sharpness of the image to be photographed prior to the exposure.